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c) Nickel/Cadmium Cells

Anode: Cadmium
Cathode: Nickel oxyhydroxide Ni(OH)2
Electrolyte: Aqueous potassium hydroxide (KOH)

In 1899, Waldemar Jungner of Sweden created the first nickel-cadmium battery. The cathode is nickel-plated, woven mesh, and the anode is a cadmium-plated net. Since the cadmium is just a coating, this cell's negative environmental impact is often exaggerated. The electrolyte, KOH, acts only as an ion conductor and does not contribute significantly to the cell's reaction.. NiCd batteries usually have a metal case with a sealing plate equipped with a self-sealing safety valve. The positive and negative electrode plates, isolated from each other by the separator, are rolled in a spiral shape inside the case. The chemical reaction which occurs in a NiCd battery is:

Half Reactions:
Cd + 2OH- -> Cd(OH)2 + 2e-
NiO2 + 2H2O + 2e- -> Ni(OH)2 + 2OH-

Overall Reaction:
Cd +NiO2 + 2H2O -> Cd(OH)2 + Ni(OH)2

This reaction goes from left to right when the battery is being discharged and from right to left when it is being recharged. The alkaline electrolyte (commonly KOH) is not consumed in this reaction.

The nickel-cadmium battery (commonly abbreviated NiCd or NiCad) is a popular type of rechargeable battery for portable electronics and toys, using the metals nickel (Ni) and cadmium (Cd) as the active chemicals. They are sometimes used as a replacement for primary cells, such as heavy duty or alkaline, being available in many of the same sizes. In addition, specialty NiCd batteries have a niche market in the area of cordless and wireless telephones, emergency lighting, as well as power tools.

Due to their beneficial weight/energy ratio as compared to lead based technologies and good service lifetimes, nickel-cadmium batteries of large capacities with a wet electrolyte (wet NiCds) are used for electric cars and as start batteries for aeroplanes.

It is sometimes claimed that NiCd batteries suffer from a so-called "memory effect" if they are recharged before they have been fully discharged. The apparent symptom is that the battery "remembers" the point in its charge cycle where recharging began and during subsequent use suffers a sudden drop in voltage at that point, as if the battery had been discharged.

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d) Nickel/Metal Hydride (NiMH) Cells

Anode: Lanthanide or Ni alloys
Cathode: Nickel oxyhydroxide
Electrolyte: Potassium hydroxide

This sealed cell is a hybrid of the NiCd and NiH2 cells. Previously, this battery was not available for commercial use because, although hydrogen has wonderful anodic qualities, it requires cell pressurization. Fortunately, in the late 1960s scientists discovered that some metal alloys (hydrides such as LiNi5 or ZrNi2) could store hydrogen atoms, which then could participate in reversible chemical reactions. In modern NiMH batteries, the anode consists of many metals alloys, including V, Ti, Zr, Ni, Cr, Co, and Fe.

Except for the anode, the NiMH cell very closely resembles the NiCd cell in construction. Even the voltage is virtually identical, at 1.2 volts, making the cells interchangeable in many applications.
Here are the cell reactions:

Half Reactions:

MH + OH- -> M + H2O + e-
NiOOH + H2O + e- -> Ni(OH)2 + OH-

Overall Reaction:

NiOOH + MH -> Ni(OH)2 + M E = 1.35 V

The anodes used in these cells are complex alloys containing many metals, such as an alloy of V, Ti, Zr, Ni, Cr, Co, and (!) Fe. The underlying chemistry of these alloys and reasons for superior performance are not clearly understood, and the compositions are determined by empirical testing methods.

A very interesting fact about these alloys is that some metals absorb heat when absorbiong hydrogen, and some give off heat when absorbing hydrogen. Both of these are bad for a battery, since we would like the hydregen to move easily in and out without any energy transfer. The successful alloys are all mixtures of exothermic and endothermic metals to achieve this.

The electrolyte of commercial NiMH batteries is typically 6 M KOH

The NiMH cell does cost more and has half the service life of the NiCd cell, but it also has 30% more capacity, increased power density (theoretically 50% more, practically 25% more). The memory effect, which was at one time thought to be absent from NiMH cells, is present if the cells are treated just right. To avoid the memory effect fully discharge once every 30 or so cycles. There is no clear winner between the two. The better battery depends on what characteristics are more crucial for a specific application.

NiMH batteries perform better with high drain electronic devices like digital cameras and other common electronic devices, while for high torque power tools and other devices that require fast discharge rates, NiCd can outperform NiMH.

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e) Nickel Zinc Cells

Anode: Zinc
Cathode: Nickel oxide
Electrolyte: Potassium hydroxide

The combination of nickel and zinc is very interesting because of the low cost and low toxicity of the constituents. There have been many technical obstacles, but a string of recent patents and a commercial start-up based on a KOH electrolyte holds great promise for applications where light weight is an issue.

The nickel/zinc battery uses zinc as the negative electrode and nickel hydroxide as the positive.

The discharge reactions are:

Half Reactions:

Zn + 2OH- -> Zn(OH)2+ 2e
2NiOOH + 2H2O -> 2Ni(OH)2 + 2OH-

Overall Reaction:

2NiOOH + Zn + 2H2O -> 2Ni(OH)2 + Zn(OH)2 E = 1.73 V
Zinc on the electrolyte tends to redeposit unevenly on anode, severely reducing efficiency.